Pressure accumulator circuit for hydraulic presses



April 1953 o. HOFFMANN PRESSURE ACCUMULATOR CIRCUIT F OR HYDRAULIC PRESSES Filed Nov. 26 1948- Y n i :am

Hill!! Patented Apr. 28, 1953 1 i PRESSURE ACCUMULATOR CIRCUIT FOR HYDRAULIC PRESSES Otto Hofimann, Buffalo, N. Y., assignor to Lake Erie Engineering Corporation, Buffalo, N. Y., a corporation of New York Application November 26, 1948, Serial No. 62,038

10 Claims. 1 This invention relates to improvements in a hydraulic circuit for hydraulic presses.

A primary object of the invention is to provide a hydraulic circuit particularly for heavy duty presses and which provides such. a press 1 Another important object is to provide such a hydraulic circuit for a heavy duty press havin operating mechanisms which can be compactly arranged on the press, thereby to provide a selfcontained heavy duty press unit.-

Another object is to provide a hydraulic system for a press in which the hydraulic circuit is precharged to provide a reserve of potential energy operatively released during the pressing or workin stroke of the press ram and recharged during all other phases of the operating cycle of the press.

Another object is to provide such a prechar'ged hydraulic circuit which requires only a relative ly small amount of power input which is em ployed essentially to recharge the circuit.

Another object is to provide such a hydraulic circuit; in which shock is eliminated, particularly when the movable-die engages the workpiece and on the return stroke of the ram.

Another object is to provide a hydraulic circuit fora hydraulic press in which the maximum pressure is applied during the entire pressing or'working stroke.

' Another object is to provide'such a'hydraulic circuit for hydraulic presses in which the working pressure'can be readily adjusted to suit the particular work to be done. j I

Another object is to provide such a hydraulic circuit for hydraulic presses in which the desired working pressure is never exceeded.

Another object is to provide such a hydraulic circuit in which theraising of the ram at the end of the pressing stroke can be initiated eitherby a limit switch or. by the pressure applied to the workpiece. r

v Another object is to provide such a hydraulic circuit which can be embodied in a press and adapted for selectively operatin the-press on a cycle which permits of inching under manual control, semi-automatic or single cycle, fully automatic, or multiple cycle. i

Other objects and advantages of the invention will appear from the following description and drawing in which Fig. 1 is a diagrammatic .representation of a hydraulic circuit embodying the present invention and applied to a heavy. duty high speed press and showing a simplified electrical control circuit adapted for the semi-auto.- matic or single cycle operation thereof,.while Fig. 2 is a sectional .view of the throttle valve. The hydraulic circuit embodyin the present invention can be used in connection with vari-.- ous kinds of presses and for the purpose of illus tration is shownas used in connection with a hydraulic press of-the downward pressure type. This press is shown in simplified diagrammatic form. as having a lower stationary platen l. cooperating with a vertically movable upper platen 2 secured to the lower end of a ram 3. This ram has an .enlarged piston head 4 which works in the bore 5 ofaclosed stationary main'cylinder 6. 'A closed reservoir or tank .1 is mounted on the upper end of the main'cylinder 6, communi cation between this reservoir and-the upper end of the bore 5 of the main cylinder being provided by an opening '8 under control of an up wardly closingprefill valve 9. This valve 9 is actuated through a piston rod l0 projecting up wardly from the 'prefill j valve" "and: connecting with a piston l which works ina closed stationary cylinder Hi, this stationary cylinder being arranged in the reservoir ortank. 1. Both the piston H and the main ram 3- are of the double acting type. However it .willbe seen that a'sep-' arate pull back means could be provided for the main piston 4 and that the closed lower end of the cylinder b provides such atpull back means since liquid forced into the lower end of this" cylinder raises the-piston 4, The liquid in'the tank 1 can be maintained under 'slightpressure' by pressurizing the air in the upper part of this tank. I Y c In accordance with the invention .the hy-" draulic circuit is shown as comprising a main pump 15 of any suitable type, driven by'an:elec--.

tric motor I 6. The main pump 15 delivers liquid under pressure through a pump discharge line I! which is operatively connected through a check valve l8 to a line [9 connecting with a' plurality of gas filled bottles. 20. Each of thesebottles 20 contains a quantity ofan inertgas such as nitrogen which is compressed to a rela-= tively high initial pressure. The bottles 20 can be of any suitable type which establish pres-'- sure communication between the gas in the bottles and the liquid in the line I9, it being preferred to employ such a bottle in which the gas and liquid are separated by a movable pressure transmitting member. Bottles of this type are commercially available.

The main pump is unloaded during starting of the motor I6 and for this purpose branch lines 2I and 22 connect the main pum discharge line I I with a normally open spring loaded pump unloading valve 23, which when open places the branch line 22 in communication with the valve outlet line 24 leading to the reservoir or tank 'I. This return line 24, as well as similar return lines for other valves to be described hereinbelow, is represented by an arrow. The valve 23 is of any suitable conventional ,form in which control pressure acting against a spring closes the valve and cuts off this flow of liquid from the branch line 22 to the return line 24. A liquid supply line 25 connects the main pump 15 with the reservoir or tank I.

The pressure for so closing the spring loaded pump unloading valve .23 is supplied from a pilot pump 26 driven through a belt 28 from the pump motor It and the discharge line 29 which is operatively connected by a branch line 30 with the pump unloading valve 2'3. This branch line 38 is provided with a restriction represented by the numeral 31., this restriction providing a time lag between the start of the pump motor 16 and the closing of the pump unloading valve 23 A liquid supply line 2] connects the pilot pump 26 with the liquid supply line 25 leading from the reservoir or tank I. A branch line .291 having a restriction 2292 connects the discharge line 29 with the reservoir or tank and provides a means of relieving the pilot pressure in the discharge 1ine29 when the motor 155 is stopped.

maximum operating pressure in the line 18 can be adjusted to the value desired. For this purpose an adjustable pressure relay valve 32 is arranged in a branch line 33 and connected at one end to the line is and at its opposite end being operatively connected with a normally closed spring loaded pump unloading valve M which when open places the branch line 22 in communication with the valve outlet line 35 leading to the reservoir or tank is The valve 31 is similar, except for being "normally closed, to the valve 23 in that pressure admitted through the line 33 acts against a spring to open the valve 3 4 and permit the now of liquid from the bunch line 22 to the return line 35. Thus, when the pressure in line I9 exceeds the setting of the pressure relay valve 32, pressure is applied to the pump unloading valve 3 "to open this valve and permit the :main pump 15 to unlead through the lines 21, 22 and 35 "I'he discharge line 29 of the pilot pump :26 is provided with a relief valve 86 having a return lin 3*! to the tank or reservoir 1 and operating to prevent the outlet pressure of the pilot pump 26 from rising above a predetermined value.

The prefill valve 9 is actuated by pilot pressure derived from the pilot pump 26. For this purpose a branch line 38 connects the pilot pump discharge line 29 with the inlet of a four way pilot solenoid valve 89. When the winding 49 of this "solenoid valve is de-energized its inlet line 38 is in communication with a control line M leading from one port .or said pilot valve to the stationary cylinder 12 above the piston 11 and at the same time a control line 42 leading from another port of said pilot valve to the cylinder I2 below the piston II is in communication with a return line 43 leading to the tank or reservoir I. Conversely, when the winding 450 of the four way pilot solenoid valve 39 is energized, its inlet line 38 is in communication with the line 42 and the line H is in communication with the return line 43.

In order to admit liquid under working pressure to the main cylinder 6 above the piston 4 of the ram 3, a branch line 45 having a restriction 38 connects the line I9 with the inlet 41 of a normally closed spring loaded working pressure valve 48. A line 5I connects a port 49 of this valve with the-main cylinder 5 above the piston I-of the ram 3. A return line 55 connects another port '52 with the reservoir or tank I. When the valve 48 is closed there is no communication between the branch line 45 and the line '5I but communication is provided between the ports 49 and 52 so that liquid from the line 5| is able'to flow through the return line 50' to the reservoir or tank .I. To establish com munication between lines 25 and 51 the valve 48 '5 opened against the urging of its spring by pilot pressure admitted through a line 53 operatively connecting this valve "With the line -42. 50 opening the valve 48 breaks communication between the lines 51 and Si]. The working pressure valve 48 may be of any suitable convention'aliform.

To permit the ram 3 to descend, the liquid in the main cylinder :6 below the piston 4 of the ram is relieved through the line .55 which connects with a spring loaded throtting control valve .58. this liquid flowing through this valve too line 58 containing a check valve 5% and leading to a port .68 of a spring loaded pull back valve 5i. This .liquid flows from the port v62 of this pull back valve through a return .line 63 leading to the reservoir or tank 1.

The throttling valve 56 is illustrated in detail in 2 and is shown as being in the form of a two way check valve comprising a stationary casing having a horizontal central partition H3 providing an upper chamber I-Hi and a lower chamber H5. The line 58 connects with the :upper chamber 114 and the lower chamber H5 is connected by the line with the lower end of the bore :5 of the main cylinder 6- A spring loaded check valve I It in the upper chamber IM seats against .an opening Hi9 through the partition H3 to prevent flow of liquid from the up per chamber I14 to the lower chamber H5 but to permit arestricted reverse flow when the pressure of its spring is overcome. A spring loaded check valve I28 in the lower :chamber I I5 seats against an opening I2-I through the partition .II:3 to prevent flow of liquid from the lower chamber H5 to the upper chamber II L This check valve I23 vhas a stem 22 connected with a pie-- ton 123 in a cylinder I 25 formed in the casing of the throttling valve 56, this cylinder I24 communicating with the line 54. The check valve IIB permits a restricted flow of pull back liquid.

into the bore 5 and the flow of pull back liquid in the opposite direction is resisted by'the spring loaded check valve I20, the spring being adjusted so as "to close this check valve andcounterba lance the weight of the ram 3 and its associated moving parts but opening to provide a throttled' relief of pull back liquid from the b0re'5 when working pressure is applied to the top of the pisten 4'. The spring loaded check valve I20 "is opened against the urging of its spring by pilot pressure admitted through the line t l operatively connecting the throttling control valve,:5-,with the pilot. pressure line 29 through "a normally closed.v slow" down solenoid valve 65 having; a winding designated at 66. When the winding 5.6 is energizedthe solenoid valve 65 opens and places lines 29 and 64 in communication with each other thereby to apply pilot pressure to the upper faceof the piston l23. This pilot pressure thereby opens the spring loaded check valve l2llagainst the urging. of its spring, thereby to place lines 55 and 58 in communication with each other.

Thus the throttling control valve 56 serves the purpose of holding up the moving weight when the main pump I5 is in neutral or not running, such moving weightv being balanced by spring pressure against the spring loaded check valve l2 0. The throttling valve 56 serves'the .further purpose of slowing" down the speed ofthe ram 3 for c the working or pressing stroke of the ram. During the high speed approach of the closing stroke, the spring loaded check valve I of the throttling valve 56 is held open by pilot pressure which acts on the piston I23. against the urging of the spring of this check valveiZlI. Slowing down the descending ram 3 to pressing speed is accomplished by withdrawing the pilot pressure against the piston I23 of the throttling control valve 56 and forcing the liquid in the line which is under a pressure exceeding that of the spring pressure of the spring loaded check valve I20 through this check valve against its spring pressure and into the line 58.

The spring loaded pull back valve 6| also has a port 68 connected by a line 59 having a restric-' tion l0 with the line [9. Thi spring loaded pull back valve 6| is'operated against the urging of its spring by pilot pressure admitted through a line ll operatively connecting this valve with an outlet of a normally closed return solenoid valve 13. This valve 73 has an inlet connected by a line 75 with the pilot pump discharge line 29 and also has an outlet connected by a return line 18 leading to the reservoir or tank I. The return solenoid valve 13 has a windin 19 which, when de-energized, places lines H and E8 in communication with each other and cuts off the pilot pressure line 'l5, and, when energized, places lines 15 and 1| in communication with each other and cuts o'fi'the return line 78.

' To limit the upward movement or overtravel of the ram 3, a normally open shut-oh valve is arranged ina line 81 which at its opposite ends connects with line 58 and which bypasses the check valve 59'. This normally open shut-ofi valve 80 is closed. by. a cam bar 82 moving with the" movable platen 20f the press. The cam bar 82 projects upwardly from its mounting and, in the elevated position of the press ram, is arranged to operatively engage and close the shut-off vlav'e- The electrical circuit is shown as including three limit switches 85, 86 and Ill actuated, respec tively, by three vertical cam rods 88, 89 and 90.

moving with the movable platen 2 of the press.

The cam rod .88 projects upwardly from its mount ing and, in the elevated position of the press ram',

is arranged. to engage and open the initially open returnlimit switch 85. The rod 89 projects down-.

8t into engagement withits fixed contact 83;: The rod fillprojects downwardly from its-mounting and, in the extreme depressed position of the ram; is arranged to engage and open the normally closed down or maximum stroke limit switch 81.

One side of the main power line connects with the winding 49, 19 and 66 of each of the sole.-. noid valves 39, 13 and 5, respectively, and also connects with the winding 96 of a holding relay S8. The other end of the winding 96 of this holding relay 9% is connected by a wire 99 withthe fixed contact of the down o maximum stroke limit switch 81. The other end of the winding .40 of the close solenoid valve 39 is connected by a wire Hill with the lower fixed contact 93 of the slow down limitswitch 86. The other end of the winding E9 of the return solenoid valve 13 is connected'by a wire Hll with the movable contact of the return limit switch 85. The other end of the winding 66 of the slow down solenoid valve 65 is connected by a wire Hi2 with the upper fixed cone tact 92 of the slow down limit switch 86.

The other side I93 of the main power line is connected by a manually operable normally open close button I04 with a wire H35 connecting with the movable contacts of the slow down limit switch 86 and down or maximum stroke limit switch 81. This manually operable close button Hid also connects this side I03 of the main power line with a wire I06 connecting with the normally open fixed contact ll of the relay 98. The armature l 08 of this relay 58 is connected through the wire me directly with the side I03 of the main power line and its normally closed fixed contact I I0 is connected through a wire H I with the fixed contact of the return limit switch 85.

The above electrical circuit is adapted for single cycle or semi-automatic operation of the press wherein the operator a'ctuate the close button I04 to effect each complete operating cycle. This electrical circuit is shown in simplified form for the purpose of illustrating one operative embodiment of the invention and it will be understood that in actual practice the press would have an q electrical control circuit, permitting of inching and-full automatic operation as well as the single cycle operation shown, It will also be understood that the hydrauli circuit can include such unessential but desirable equipment as handval-ves, relief valves, oil filters,-

; a; hydraulically operated stripper mechanism' which is operatively connected into the hydraulic circuit'.'

g Operation At the start -of-operation it is desirable that thefull load upon'th'e'n'iotor I6 be applied grad ually and to this end the main pump 15 is unloaded when the motor is started. Thus, upon energizing the motor IS, the discharge 'from the main pump l5'passes through the main pump discharge line ll, bran-ch lines 2|.and 22, open spring loaded pump unloading valves 23 and through 'its' return line, designated by the ar-' row 24, to the reservoir or tank 1 which is 'con- -nected with the supply line 25- leading to the inlet of the main pump. At this time the liq uid discharged by the main pump l5 cannot enter the spring loaded pump unloading valve 34 through the branch line 22 since this valve is" closed. I r

was or tank 1.

accuses $tal21ll tfl motor I3 also the $12101, pump 26 and the outlet pressure from? this pump is applied from its discharge line 2-9 through the branch line so and restriction or to the now open spring loaded pump unloading valve 23* This closes the valve 23, the restriction 3! providinga sufliclent time lag for the motor it to reach running speed before full load: is applied to it. With the pump unloading valve 23 closed; the pressure in the main pump discharge line H against the check valve I8 opens this check valve so that fluid is pumped into the line f9 for operation of the press including recharging the bottles 20.

In. the event the pressure in the line l9 excards the maximum dtermined by the setting oi the adjustable pressure relay valve 32, this valve admits liquid through its outlet line 33 to the spring loaded pump unloading valve 34 to open this valve against the urging of its spring. With this valve 3'5 s'o opened, liquid from the main pump discharge line [7 passes through the branch lines 2'! and 22- to the returnliiie 35 and is thereby returned to the res- At this time when the valve 30 is so opened the other spring loaded pump unloading valve 23 is closed. Thus the spring loaded pump unloading valves 34 and 23 together with their associated lines provide a hy draul-io circuit which serves primarily to control the pressure in the line H and secondarily to unload the main pump l5 when starting the motor Hi.

The operator then places the workpiece onthe stationary lower platen I and manually closes the normally open close button me. This establishcs a circuit from the side E03 of the main power line through close button I04, wire "l5, closed. down or maximum strokelimit switch 81-, winding 96 of holding relay 98 to the other side 95. of the main power line. The armature I08 of the now energized holding relay 9-8 is drawn into engagement with its fixed contact I01 thereby to establish through the wires H19 and "36 a bypass around the manually onerated close button IM and seal the circuit just described until broken at another point.

With the holding relay 58 so closed current passes trom the side H33 of the main power line through wire H39, armature I98 andfixed contact Hi1 of the holding relay 98, wires. I66 and. "15,-. movable contact 91 and fixed contact 92-. oi the slow down limit switch to, wire 12, and winding 85 oi the normally closed. slow down. solenoid valve to the other side 95' of. the main power line. Energi'zation of the slow down solenoid valve 65 causes this valve to open and admit pilot pressure to work against the piston F23 oi the spring loaded. ohecls. valve 12 0 oil the throttling control valve 56 and thereby open: this check valve to place lines 29 and 64 in free flow commu nication with each other. Pilot pressure so applied to the piston N3 of the spring loaded control valve 120' causes this valve to open against the urgiri'goi its spring.

The now opened control valve 58- places the line 55 incommunication with the. line 58 and: permits pull back: liquid to flow from the main cylinder 6 below the piston 4 through lirle 55,

open control: valve 55, line 58 past the check valve:- 59-, open spring loaded. pull back valve 6! and return line 53, the latter returning the pull.

batch liquid to the reservoir or tank 1. It will be noted that at the beginning of the descent of the rain 3 the shut ca valve so is completely closed so that liquid flowing in the line 58 to ward the pull back valve 5! must ass through the check: valve Sd. However, as the ram lowers the" shut oii? valve at is permlttedto open to a. fully opened position by reason of the cam bar 82 lowering" with the rain and this cam bar disengaging: this valve so that the pull back liquid being exhausted from the main cylinder 6 can either pass the check valve 59 or flow through the now opened shut off valve During the descending movement of the" ram 3 prefill. liquid is supplied to the main cylinder 5 above the piston 4 from the reservoir or tank i, the pre'fill valve 9 being open to permit such flow. The prefill liquid in this reservoir or tank I can be under prowl-re or not, as may be required to secure the desired prefllling speed. The preflll valve 9 is held open. at this time through pilot pressure applied to the upper side of its. piston H from the pilot pump discharge line 29, inlet line 38', and four way pilot solenoid valve 39 which is de energiz'ed and. has has its inlet line 3-8 in communication with its line 4! and its line 42 in communication with its return line 83 so that the" liquid under the piston H can escape through the line 42 valve 39 and return line 43- to the tank or reservoir 1.

As soon as the ram 3 starts its descent it sets up the electrical circuit for return of the ram-.- This is effected by the cam rod 88- which on the start of the descent of the ram 3 released the initially open return limit switch sothat this switch closes However; at this time: the closing ofthis return limit switch 85 does not energize the winding it of the return solenoid valve 13* because at this time" the holding relay 9-8 is ener gized so that its armature 89% is out of contact with its fixed contact H0 and therefore aclosed circuit through the winding T9 is not established Since this winding of the return solenoid It is" not energized, pilot pressure continues to be cut oft from the spring loaded pull back valve EH so that pull back liquid continues to be discharged from its return 1ine$3 to the reservoir or tank 1;

This descent of the ram 3' constitutes the high: speed approach of the ram to the workplace and is sloweddown just before the die (not shown) carried by the upper movable platen Z engages the workpiece. is cheated by the cam rod to engaging the slow down'switch 85 and moving its movable contact 8i out oi. engagement: with the fixed contact 92 and into engagement with the fixed contact 93. Breaking the contact between the contacts 91 and 92 breaks the circuit through the winding [it of the normally closed. slow down. solenoid valve 6'6 thereby cutting off pilot pressure to the spring loadedcontrol valve 55. Such withdrawal of the pilot pressure from the control valve 56 permits the spring loaded check valve I29 of this control valve partially to close and thereby reduce the speed of descent of the ram by throttling the escape of pull back liquid from. below the piston d. The spring loaded valve m does not completely close at this time inasmuch as the pressure of the liquid in: the line 55' exceeds that of the spring pressure against this spring loaded valve E28, this preponderating liquid pressure in the line all being due to the application of an increased force upon the upper end of the ram 3.

This increased pressure upon the upper end of the ram 3 is applied simultaneously with this throttling oi; the escape of pull back liquid by Slowing the ram down 9,, the :engagement ofthe contacts 9|. and 93 of the slow downlimit switch 86. This engagement establishes a circuit through the winding 40 of the four way pilot solenoid valve 39. Energization of this winding 40 operates the four way pilot solenoid valve 39 to break communication between the pilot pressure inlet line 38 and the line 4|, establish communication between this line 4| and the return line 43 to the tank I, and establish communication between the pilot pressure inlet line 68 and the line 42. Accordingly, pilot pressure is applied through line 42 to the under side of the piston II to raise and close the prefill valve 9, the-liquid above this piston escaping through the line 4|, valve 39 and return line 43 to the reservoir or tank I.

Pilot pressure so established in line 4-2 is also transmitted through branch line 53 to open the normally closed spring loaded working pressure valve 46. Opening this valve 48 permits liquid from the line I9 to flow through the branch.

line 45, restriction 46, valve 4 8 and through line 5| to the cylinder 6 above the ram. The restriction 46 provides a suificient time lag to permit the prefill valve 9 to be closed before the liquid under working pressure is admitted to the main cylinder 6 through line 5|.

. It will particularly be noted that the liquid so admitted to the main cylinder 6 from the line I9 is under the maximum pressure built up in the bottles 28 and also in accordance with the setting of the adiustable'pressure relay valve 32. It will also be noted that this pressure in the bottles 28 is not utilized until the ram so starts its pressing strok so that the full'reserve of to re-engage the armature with its fixed con-v tact I I8.

Breaking the contact between the armature I88 and fixed contact Ili'I-of the relay'98 operates to cutoff the flow of liquid under working pressure to the upper side of the ram 3 and also to open the; prefill valve 9 so that upon thereafter raising the ram 3 by introducing pull back liquid under pressure to the under side of its piston 4 the liquid above the ram 3 can enter the reservoir or tank I through the passage 8.

Thus, breaking the contact between the armature I68 and fixed contact I01 of the relay 98 breaks the circuit through winding 4|] of the four way pilot solenoid valve 39 thereby to establish communicationbetween the lines 42, 53, and the return line 43 to the tank or reservoir I, andto establish communication between the pilot pressure inlet line 38 and the-line 4|. Accord ingly, pilot pressure is relievedfrom application tothe normally closed spring loaded working pressure valve 48 by reason of the line 53 being so'c'onnected through the line 42 'to the return linev 43 to the tank or reservoir I. Relieving the pilot pressure on the valve 48 permits this valve to shift thereby to cut oil communication between the working pressure inlet line.45 leading from line I9 and the line 5| leading to the main cylinder 6, and establishing communication between line 5| ,from' the main cylinder and the return line 56 to the tank or reservoir I. Ac:

cordingly the application of working pressure to Y the upper end of the ram 3 is cut ofi, and the existing pressure therein relieved. At the same time pilot pressure is admitted to the cylinder I2 above the piston II to force this piston downwardly and open the prefill valve 9, the liquid below this piston escaping through the line 42 and return line 43 to the reservoir or tank 'I. This action cannot occur however until the pressure above ram 3 is relieved.

Making contact between the armature I08 and fixed contact III! of the relay 98 operates to admit pull back liquid under pressure to the main cylinder 6 below the piston 4 and thereby raise the ram 3.

Thus, this engagement of the armature I68 and fixed contact III) of the relay 98 closes a circuit from the side I03 of main power line through wire I09, armature I08, fixed contact III'I, wire III, closed return limit switch 85, wire IIII, and winding I9 of the normally closed return solenoid valve I3 to the other side 95 of the main power line. Energization of thisnormally closed return solenoid valve I3 places the pilot pressure branch line I5 into communication with the line II leading to the spring loaded pull back valve 5|. The pilot pressure. so applied to this valve 6| operates this valve against the pressure of its spring to .break communication between the line 58 and return line 63 to the reservoir or tank I and to establish communication between the line 58 and the branch line 69 leading from the line I9. When the valve 6| is so operated, liquid under the maximum working pressure passes from line I 9, through branch line 69 and restriction I0, valve 6|, line 58, bypass line BI and the now open shut-off valve 86, throttling control valve 56 and line 55 to the main cylinder 6 below the piston 4. Accordingly, the ram 3 is lifted or returned, the liquid'above this ram flowing past the open prefill valve 9 into the tank or reservoir I. It will be noted that the restriction I0 in the branch line 69 provides a suflicient time lag to permit the working pressure against the top of the ram 3 to be relieved and the prefill valve 9 to open before pull back liquid under pressure is so applied to the under side of the piston 4.

, The ram 3 continues to rise until the cam rod 88 reengages and opens the return limit switch 85, this engagementbeing slightly in advance of the engagement of the cam bar 82 with the shutoff valve 80. Opening the return limit switch 85 de-energizes the winding I9 of the return solenoid valve,I3 thereby to break communication'between munication between lines 58 and 63 IE-GStfibllShGS the condition of the spring loaded pull back valve 6| for another cycle of operation.

,The cam bar 82 then engages and closes the shut-oil valve due to overtravel of the ram 3 to re-establish the condition of this valve for another cycle of operation, the weight of the moving parts again being supported by the spring loaded check valve I20 in the throttling control valve 56 11 as previously described. This cycle is then repeated as above described. It will be noted that at the end of each cycle all of the solenoid valves are lie-energized preparatory for the next operating cycle To illustrate the advantages of the hydraulic circuit, particularly in respect to providing a heavy duty press with a short operating cycle, a GOO-ton press embodying this circuit has been constructed and which works at 20 strokes per minute,- this including a closing stroke of 2 inches, a pressing stroke of .5 inch and also an allowance of time for loading and unloading which takes up almost half of the three-second cycle.- This extremely rapid cycle for a press of this capacity is due primarily to the application of stored power at a particular point in the operation of the press, namely, during its pressing or working stroke. Thus, the pressure stored in the bottles 2% is ap plied to the ram at the start of the 5' inch pressing or working stroke and is cut on at the end of its return stroke,- the total time for these strokes being less than half a second and these bottles being recharged during the remaining time of the operating cycle; This efficient utilization of the stored power in the bottles 20 "so provides an extremely high operating speed in a heavy duty hydraulic press with a minimum horse power input, the pressing speed of the present circuit exceeding any speed heretofore attained with presses of such capacity in hydraulic presses It provides the advantage of speed of mechanical presses with the advantages such as constant pressure during the pressing or working stroke,

of hydraulic presses, and with a minimum power input.

I claim:

1. In a selfcontained hydraulic press unit, the combination comprising a cylinder having a bore,

a piston arranged in said here, pull back means associated with said piston, a ram movable by said piston and pull back means successively through closing pressing and return strokes, a liquid reservoir in communication with said bore for introducing lprefill liquid therein during the closing stroke of said ram, 2. prefill valve operative to break communication between said reservoir and bore, a constantly operating pum ror the working liquid, a discharge line connecting the outlet of said pump with said bore for introducing working liquid therein during the press ing stroke of said said, a working pressure valve in said discharge line, a container individually associated with said discharge: line and charged with compressed gas in pressure transmitting relation with the working liquid in said discharge line between said pump and working pressure valve whereby said container is recharged through pressure derived from the working liquid suppliedby said pump, a pull back line con-necting said discharge line, between said pump and working pressure valve, with said pull loack'means for applying pull back liquid to said pull back means during the return stroke of said ram, a pull back valve in said pull back line, means ar ranged to permit the withdrawal "of pull back liquid from said pull back means during said closing and pressing strokes, means arranged to close said prefill valve and open said working ressure valve and operative at the beginning of said pressing stroke, means operative at the end of said pressing stroke and arranged to close said working pressure valve to permit recharging of said container by said pump and arranged to open said prefill valve, and means arranged to open and close said ull back valve and operative to 12 open said pull back valve at the beginning of said return stroke and to close said pull back valve at the end of said return stroke.

2. In a self-contained hydraulic press unit, the combination comprising a cylinder having a bore, a piston arranged in said bore, pull back means associated with said piston, a ram movable by said. piston and pull back means successively through closing, pressing and return strokes, a liquid reservoir in communication with said bore fol? introducing prelill liquid therein during the closing stroke of said ram, a prefill valve operative to break communication between said reservoir and bore, a constantly operating pump for the working liquid, a discharge line connecting the outlet of said pump with said bore for in troducing working liquid therein during the pressing stroke of said ram, a working pressure valve in said discharge line, a container individu'ally associated with said discharge line and charged with compressed gas in pressure transmitting relation with the workil'ig liquid in said discharge line between said pump and working pressure valve whereby said container is re charged through pressure derived from the working liquid supplied by said pump, a first pull back liquid line connected to said discharge line between said pump and working pressure valve, a second pull back liquid line connected to said pull back means for alternately withdrawing pull back liquid therefrom during said closing and pressing strokes and introducing pull back liquid thereto during said return stroke, a return line, pull back valve means operatively connecting said first and second pull back liquid lines and return line, means arranged to actuate said pull back valve means to establish communication between said second pull back liquid line and said return line during said closing and pressing strokes and to establish communi cation between said first and second pull back liquid lines during said return stroke, a throttling control valve in said second pull back liquid line, means arranged to open said throttling control valve during said closing stroke, means arranged to close said throttling control valve to a throttling position during said pressing stroke to slow down the speed of said ram during said pressing stroke means arranged to close said prefill valve and "open said "working pressure valve and operative at the beginning of said pressing stroke,

and means operative at the end-of said pressing stroke and arranged to close said working pressure valve to permit recharging of said container by said pump and also arranged to open said prefill valve.

3. In a press including a cylinder having a bore, a rain arranged in said bore and movable through closing and pressing strokes, a liquid reservoir arranged to communicate with said bore for introducing prefill liquid therein during the closing stroke of said ram, and a prefill valve operative to establish and break said communication between said reservoir and bore, the combination therewith of a hydraulic circuit, comprising a constantly operating pump for the working liquid, a discharge line connecting the outlet of said pump with said bore for introducing working liquid therein during the pressing stroke of said rain, a working pressure valve in said discharge line, a container charged with compressed gas in pressure transmitting relation with the working liquid in said discharge line between said pump and working pressure valve whereby said container is recharged through pressure derivedfrom the working liquid supplied by said pump, -a pilot valve having a pair of lines connected with said prefill valve to actuate said prefill valve, and a line connecting one of said pair of lines with said working pressure valve to actuate said working pressure valve in response to the actuation of said prefill valve. I

4. In a press including a cylinder having a bore, a ram arranged in said bore and movable through closing and pressing strokes, a liquid reservoir arranged to communicate with said bore for introducing prefill liquid therein during the closing stroke of said ram, and a prefill valve operative to establish and break said communication between said reservoir and bore, the combination therewith of a hydraulic circuit, com-' prising a-constantly operating pump for the working liquid, a discharge line connecting the outlet of said pump with said bore for introducing working liquid'therein during the pressing stroke of said ram, -a working pressure valve in said discharge line, a container charged with compressed gas in pressure transmitting rela-- tion with the working liquid in said discharge line between said pump and working pressure valve whereby said container is recharged through pressure derived from the working liquid supplied by said pump, a pilot valve having a pair of lines connected with said prefill valve to close and open said prefill valve at the beginning and end, respectively, of said pressing stroke, and a line connecting one of said pair of lines with said working pressure valve to open and close said working pressure valve through power derived from said pilot valve. 1

' 5. In a press including a main cylinder having a bore, a ram arranged in said bore and movable through closing and pressing strokes, a liquid reservoir in communication with said bore, a normally open prefill valve operatively arranged between said reservoir and bore, and a piston and cylinder mechanism arranged to op erate said prefill valve to close said prefill valve and thereby break communication between said reservoir and bore, the combination therewith of a hydraulic circuit comprising a constantly operating pump, a' discharge line connecting the outlet-of said pump with said bore for introducing working liquid therein during the press-' ing stroke of said ram, a normally closed work-- ing pressure valve arranged in said discharge line, a container charged with compressed gas in pressure transmitting relation with the working liquid in said discharge line between said pump and working pressure valve whereby said container is recharged through pressure derived from the working liquid supplied by said pump, and means arranged to open said working pressure valve in response to the closing of said prefill. valve, comprising pilot valve having its inlet connected to a source of fluid under pressure, a line connected to the outlet of said pilot valve and operatively connected with said piston and cylinder mechanism, a second line connected to said last'line and operatively connected tosaid working pressure valve for opening 1 the same and means arranged to open said pilot valve,

6. In a press including a main cylinder having a bore, a ram arranged in said bore and movable through closing and pressing strokes, a liquid reservoir in communication with said bore, a normally open prefill valve operatively arranged between said reservoir and bore and a double acting piston and cylinder mechanism arranged said pump. and working pressure valve whereby said container is recharged through pressure derived from the working liquid supplied by said pump, and means arranged to open said spring loaded working pressure valve in response to the closing of said prefill valve, comprising a solenoid valve having an inlet, two ports and a drain outlet, means connecting said inlet to a source of fluid under pressure, a first line connecting one of said ports to said piston and cylinder mechanism for applying fluid against one side of said piston to open said prefill valve and also for withdrawing fluid from said one side of said piston to permit said prefill valve to close,

a second. line connecting the other of said ports to said piston andcylinder mechanism for applying, fluid against the other side of said piston to close said prefill valve and also for Withdrawing fluid from said other side of said piston to permit said prefill valve to open, the normal condition of said solenoid valve being such as to establish communication between said inlet and said first line and to establish communication between said drain outlet and said second line,"

a third line connecting said second line operatively with said spring loaded working pressure valve for opening it against the urging 'ofits spring, and electrical means arranged to operate said solenoid valve thereby to establish communication between saidinlet and said second line and between said drain outlet and said first line whereby said prefill valve is closed and said spring loaded working pressure valve is opened,

7. In a press including a cylinder having a bore,- a piston arranged in said bore, pull back means associated with said piston, a ram movable by said piston and pull back means successively through closing, pressing and return strokes, a liquid reservoir in communication with said bore during said closing and return strokes, a prefill valve operative to break communication between said reservoir and bore during said closing stroke, a constantly operating pump 'for the working liquid, a discharge line connecting the outlet of said pump with said bore for introducing working liquid therein during said pressing stroke, a container charged with compressed gas in pressure transmitting relation with the working liquid in said discharge line whereby said container is recharged through pressure derivedfrom the working liquid supplied by said pump, a spring loaded pull back valve having an inlet connected with said discharge line, a return line' connected with said pull back valve, pull back liquid lines connecting said pull back valve with said pull back means and normally establishing communication between said return and pull back liquid lines, a control line operatively connecting said pull back valve with a source of fluid under pressure for actuating said pull back valve against the urging of its spring to break the, normal communication between said return and. pull back-liquid lines and to establish communiaccuses 15 cation between said inlet and said pull back liquid lines, a normally closed control valve in said control line, and means arranged to open said control valve at the beginning of said return stroke.

8.111 .a press including a cylinder having a bore,- a piston arranged in said bore, pull "back means associated with said piston, a ram mov ahleby said piston and pull back means sue cessively through closing, pressing and return strokes, a liquid reservoir in commmrication with said bore during said closing and return strokes, a prefill valve operative to break communication between said reservoir and here during said clue-'- lng stroke, a constantly operating pump for the working liquid, at discharge line connecting the outlet of said pump with said bore for introducing working liquid therein during said pressing stroke, a container charged with compressed gas in pressure transmitting relation with the working liquid in said discharge line whereby said container is recharged through pressure derived from the working liquid supplied by said pump, a pull back liquid line connecting said discharge line with said bore on the side of said piston opposite from said prefill valve, pull back valve means operatively associated with said pull back liquid line and arranged to permit the withdrawal of pull back liquid from said pull back means through said pull back liquid line during said closing and pressing strokes and to introduce pull back liquid into said pull back means from said discharge line during said return stroke, a spring loaded throttling control valve arranged in said pull back liquid line bea tween said pull back valve means and said pull back means and operative to permit the unrestricted flow of pull back liquid into said pull back means and operative to throttle the flow of pull back liquid from said pull back means, a

control line operatively connecting said spring loaded throttling control valve with a source of fluid under pressure for said throttling valve against the urging of its spring; a normally closed control valve in said control line, and means arranged to open said control valve during said closing stroke.

9. In a; self-contained hydraulic press unit, the combination comprising a main cylinder having a bore, a main piston arranged in said bore, pull back means associated with said piston, a ram movable by said main piston and pull back means successively through closing, and return strokes, a liquid reservoir in comnrmiication with said bore, a normally open prefill valve operatively arranged between said reservoir and bore, a double acting piston and cylinder mechanism arranged to open and close said prefill valve, the closing or said prefill valve breaking communication between said reservoir and bore, a constantly operating pump for the working liquid, a discharge line connecting the outlet of said pump with said here on the side of. said main piston adjacent said prefill valve for introducing working liquid during the pressing stroke of said rang a normally closed spring loaded working pressure valve-arranged in said. discharge line, a container charged with compressed gas in pressure transmitting relation with the liquid said discharge line between said pump and working pressure valve whereby said container is recharged through pressure derived from the working liquid supplied by said pump,- a source of fluid under pressure, a four way pilot valve having an inlet, two ports and a return line, means con-- 16 nesting said inlet with said source: a first line connecting one of said ports to said piston andcylinder mechanism for applying said fluid against one side of said piston to open said pre-;

fill valve and also for withdrawing fluid from said one side of said piston to permit said prefilli valve to close, a second line connecting the other of said ports to said piston and cylinder mechanism for applying said fluid against the other side or said piston to close said prefill valve and also for withdrawing fluid from said other side of said piston to permit said prefill valve to open. the normal condition of said four way pilot valve being such as to establish communication between said inlet and said first line and to establish communication between said return line and said second line; a third line connecting said second line operatively with said spring loaded working pressure valve vfor opening it against the urging of its spring. means operative at the beginning ci". said pressing stroke and arranged to operate said four way pilot valve to establish communication between said inlet and said sec- 0nd line and between said return line and said first line whereby said prefill valve is closed and said spring loaded working pressure valve is opened, a spring loaded pull back valve having an inlet connected with said discharge line between said pump and working pressure valve, said pull back valve also having a return line and a port connected with said pull back means and normalh establishing communication between said return line and said pull back valve port; a fourth line operatively connecting said pull back valve with said source for actuating said pullback valve against the urging of its spring to break the normal communication between said return line and said pull back valve port and to establish communication between said pull back valve inlet and said pull back valve port con-.

nected with said pull back means, a normally closed return valve. in said fourth line, means arranged to open said return valve at the ber ginning of. said return stroke, a spring loaded throttling control valve operatively arranged in the connection between said pull back valve and. said pull back means and operative to permit. the unrestricted flow of pull back liquid intosaid pull back means and operative to throttle the flow oi pull back liquid from said pull back means; a fifth line operatively connecting said spring loaded throttling control valve with said source for opening said throttling control valve against the urging of its spring, a normally closed slow down valve in. said fifth. line; and means arand thereby break communication between said.

reservoir and bore, the combination therewith of a hydraulic circuit comprising a constantly operating pump, a discharge line connecting the outlet of said pump with said bore for introducing working liquid therein during. the pressing stroke of said ram, a normally closed worlfing pressure valve arranged in said discharge line, a container charged with compressed gas in pres sure transmitting relation with the. working.

17 liquid in said discharge line between said pump and working pressure valve whereby said container is recharged through pressure from the working liquid supplied by said pump, and means arranged to open said working pressure valve in response to the closing of said prefill valve, comprising a pilot valve having its inlet connected to a source of fluid under pressure, a return line connected with said pilot valve, a first control line connected to one port of said pilot valve and operatively connected with said piston and cylinder mechanism on the side of said piston and cylinder mechanism adjacent said prefill valve, a second control line connected to another port of said pilot valve and operatively connected with said piston and cylinder mechanism on the side of said piston and cylinder mechanism remote from said prefill valve, means arranged to actuate said pilot valve to place said second control line in communication with said return line, and a third control line connected to said first control line and operatively connected with said working pressure valve for opening the same.

OTTO HOFFMANN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,054,194 Gerdau Feb, 25, 1913 1,796,936 Montgomery Mar. 17, 1931 2,185,473 Neeson Jan. 2, 1940 2,293,334 Ernst Aug. 8, 1942 2,299,686 Ernst Oct. 20, 1942 2,302,922 Tucker Nov. 24, 1942 2,338,486 Bixler Jan. 4, 1944 2,358,650 Maude Sept. 19, 1944 2,376,519 Stacy May 22, 1945 2,395,974 Muller et a1. Mar. 5, 1946 2,411,716 Ernst Nov. 26, 1946 2,415,053 Waldie et a1 Jan. 28, 1947 2,449,401 Lindsey Sept. 14, 1948 

